Electronic apparatuses have been downsized and become denser in packaging. This market trend involves printed wiring boards, to which electronic components are to be mounted, to be double-sided boards, or multi-layer boards. As such, higher density boards that can accommodate more circuits and more components are developed.
More elaborated design rules are available to a printed wiring board of higher density, which needs finer circuits than those of a conventional printed wiring board, so that a work technique for forming a finer circuit has been developed. For multi-layer boards, techniques of alignment of through-holes, via holes or circuits between layers have been developed in addition to a technique of connecting layers at finer pitches.
As an electric signal of a higher frequency is used, regulating a thickness of an insulating layer between active layers becomes a more vital factor in addition to the work accuracy of the circuits on the boards.
A conventional manufacturing method of printed wiring boards is described hereinafter. First, prepreg in stage B is sandwiched by two sheets of copper foil. The prepreg is formed by the following method: Woven fabric or non-woven fabric made of glass fiber or aramid fiber is impregnated with thermosetting resin and turned into a status called stage B. A layered structure sandwiched by metal plates at its top and bottom is hot-pressed by a hot-presser and integrally molded. After the hot-press, the copper foil is etched to form a desirable pattern, so that a printed wiring board is obtained.
FIG. 8 shows a sectional view illustrating a hot-presser used for the conventional manufacturing method of printed wiring boards. Board member 4 is formed of the foregoing prepreg sandwiched by two sheets of copper foil. The thickness of the prepreg is approx. 150 μm and the thickness of the copper foil is approx. 18 μm. Board member 4 is sandwiched by intermediate metal plates 3 made by polishing a stainless steel plate into a smooth and flat plate. The foregoing structure is referred to as the layered structure. Cushion 1 is placed on the top and bottom of the layered structure, and the layered structure is placed on carrying plate 6. As shown in FIG. 8, carrying plate 6 on which the layered structure and cushions 1 are placed is inserted between upper and lower hot-plates 7 of the hot-presser, then the layered structure is pressed by upper and lower pressing means 8.
Hot plate 7, which heats or cools board member 4, includes oil, vapor or cooling water, of which temperature is controlled as a heating medium or a cooling medium. The temperature and discharge quantity of the cooling medium are controlled, so that board member 4 is heated or cooled with a desirable temperature profile.
Pressing means 8 applies a pressure to board member 4, and a hydraulic cylinder is generally used as pressing means 8. In FIG. 8, a cylinder (not shown) is disposed at the lower side, and lower hot-plate 7 and carrying plate 6 are lifted upward by the cylinder and urged against pressing means 8 that functions as a fixed end of the upper side.
The heat and press stage includes a heating step of which peak temperature ranges 150-200□ and a pressing step of which pressure ranges 1.5-4 MPa, and this heat & press molds and hardens the prepreg in board member 4, so that the prepreg is integrated with the copper foil.
After the heat and press stage, board member 4 is taken out from the hot presser, and undergoes a boring step, a through-hole forming step where copper plating is provided in holes, and a patterning step where the copper foil is etched to form a desirable pattern before it becomes a printed wiring board.
Indirect materials such as intermediate metal plate 3, cushion 1, and carrying plate 6 are used repeatedly in the heat and press stage until they reach their own service lives. It is important for the heat and press stage to apply uniform pressure to board member 4. The printed wiring board compatible with the recent high-frequency performance is critical in particular about an accuracy of its thickness, therefore applying a uniform pressure is vital to manufacture quality printed wiring boards.
In order to obtain a uniform pressure, careful attention must be paid to parallelism of upper and lower pressing means 8 and hot plates 7 as well as dispersion of thickness of metal plate 3; however, the performance of cushion 1 is a critical factor among others. A conventional cushion employs rather thick paper such as craft paper, or elastic rubber sheet such as silicone rubber or fluoro-rubber.
A high-density printed wiring board, which has been recently demanded in the market, needs high accuracy in the manufacturing process, which thus encounters various difficulties. This situation is one of the problems to manufacture quality printed wiring boards at an inexpensive cost. The foregoing heat and press stage is strongly required to carry out the steps accurately at a high reproducibility. Respective service lives of the indirect materials such as intermediate metal plate 3, cushion 1 and carrying plate 6 directly influence the manufacturing cost, so that those service lives are also strongly required to be extended.
In the conventional manufacturing method of the printed wiring boards, paper cushion must be replaced every heat & press action because of its poor durability, and the paper cushion cannot bear a plurality of the heat & press actions. In the case of using a rubber-sheet cushion, it must be replaced before it loses the uniform pressure, namely, several tens of heat & press actions. A use of the rubber-sheet cushion in plural times results in adhering of the rubber-sheet to hot-plates 7, intermediate metal plates 3, or carrying plate 6. When the layered structure is taken out, it is thus sometimes difficult to remove cushion 1 from the layered structure.
In the case of manufacturing high-temperature resistant boards that are in progress of development, a high-temperature press at not lower than 200° C. is needed as a condition of the heat and press stage. In the case of using conductive paste or the like as interfacial connecting means for the printed wiring boards, a higher pressure than the conventional manufacturing method must be applied to the conductive paste for effecting electric connection. This will be detailed in the exemplary embodiments described later. As such, the durability of the cushion is raised as a problem, and so much severe manufacturing condition is required.
On top of that, a high-density printed wiring board, to be used in portable electronic devices recently prevailing in the market, needs to be controlled its circuit impedance at a desirable value as well as its allowable thickness within a narrower dispersion.
A problem of the cushion such as cushion property or irregular thickness will cause a wider dispersion in the thickness of the printed wiring boards, and makes the boards inadequate quality-wise.